Process for preparing platinum based electrode catalyst for use in direct methanol fuel cell

ABSTRACT

The present invention discloses a method for preparing platinum (Pt) based nano-size catalyst which is useful as an electrode catalyst of a direct methanol fuel cell (DMFC). This method includes the implementation of a reduction reaction of a platinum precursor and an optional ad-metal precursor with a reducing agent in a solvent and in the presence of a stabilizer to form a suspension containing colloidal particles of platinum or platinum/ad-metal; mixing the suspension with a co-solvent; subjecting the resultant mixture to a centrifugal treatment to form a platinum or platinum/ad-metal colloidal particle portion and a liquid portion, repeating the co-solvent mixing and centrifugal treatment to the platinum or platinum/ad-metal colloidal particle portion until the resultant liquid portion no longer contains the product of the reduction reaction; and drying the resultant platinum or platinum/ad-metal colloidal particle portion to obtain a platinum based nano-size catalyst.

FIELD OF THE INVENTION

The present invention is related to a method for preparing platinumnano-size catalyst, which can be used as the electrode catalyst(including positive electrode and negative electrode) for directmethanol fuel cell.

BACKGROUND OF THE INVENTION

The preferable electrode catalyst for use in the direct methanol fuelcell (DMFC) is the ones made of platinum or platinum alloy. There is asuitable method for preparing catalysts made of platinum or platinumalloy, which is the colloidal stabilization method. This method involvesthe implementation of a reduction reaction of a platinum precursor or aplatinum alloy precursor with a reducing agent, which is carried out ina solvent and in the presence of a stabilizer, in order to form asuspension containing colloidal particles of platinum or platinum alloy.Because the stabilizer cannot be removed easily, the colloidal platinumor platinum alloy catalyst as obtained is generally used for catalysisdirectly, or is used to deposit on a carrier then subject the carrier totemperature higher than 200° C. to remove the stabilizer, prior to itsuse in catalysis. However, the presence of the stabilizer, or theremoval of the stabilizer at temperature higher than 200° C., both haveadverse effects on the use of platinum or platinum alloy catalyst asDMFC electrode catalyst.

In U.S. Patent 2004/0087441A1, a colloidal stabilization method forpreparing platinum or platinum alloy catalyst is disclosed, whereinethylene glycol is used as the solvent, the reducing agent, and thestabilizer at the same time. Since this method uses only ethyleneglycol, it is possible to use the electrochemical oxidation methodand/or heating to a temperature less than 200° C. in air to remove theproducts generated from reduction reaction from the platinum or platinumalloy catalyst. As a result, the adverse effects on platinum or platinumalloy catalyst mentioned in the previous paragraph can be prevented. Inthis method, the resultant particle size of the platinum or platinumalloy catalyst (between 0.8 to 10 nm) can be adjusted by altering theconcentration of NaOH in the reaction compounds. The method can furthercomprise a step in which a carrier is added to the reaction mixturebefore reduction reaction is carried out, so that platinum or platinumalloy catalyst can be deposited on a carrier. The method can alsofurther comprise another step in which a carrier is immersed in asuspension resulting from reduction reaction, or coated with particlesof platinum or platinum alloy catalyst, so that carriers that has beencoated with platinum or platinum alloy catalyst can be obtained. Thedisclosures of U.S. Patent 2004/0087441A1 is incorporated herein byreference.

Although U.S. Patent 2004/0087441A1 has improved the method forpreparing platinum or platinum alloy catalyst, the method ofelectrochemical oxidation and/or heating to temperature less than 200°C. in air to remove the products generated from reduction reaction fromthe platinum or platinum alloy catalyst is still less than ideal, andcan only be used when ethylene glycol is used as the solvent, thereducing agent, and stabilizer at the same time.

SUMMARY OF THE INVENTION

A primary objective of the present invention is to provide a method forpreparing platinum based nano-size catalyst, which can be used as theelectrode catalyst for direct methanol fuel cell. This method includescarrying out a reduction reaction of a platinum precursor and anoptional additional (ad-)metal precursor with a reducing agent in asolvent and in the presence of a stabilizer to form a suspensioncontaining colloidal particles of platinum or platinum/ad-metal; mixingthe suspension with a co-solvent; subjecting the resultant mixture to acentrifugal treatment to form a platinum or platinum/ad-metal colloidalparticle portion and a liquid portion, repeating the co-solvent mixingand centrifugal treatment to the platinum or platinum/ad-metal colloidalparticle portion until the resultant liquid portion no longer containsthe side product of the reduction reaction; and drying the resultantplatinum or platinum/ad-metal colloidal particle portion to obtain aplatinum based nano-size catalyst.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the photograph of platinum black catalyst prepared inExample 2 of the present invention, which is taken under a transmissionelectronic microscope (TEM).

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention include (but not limitedthereto) the following:

1. A method for preparing a platinum based nano-size catalyst comprisingthe following steps:

a) carrying out a reduction reaction of a platinum precursor and anoptional ad-metal precursor with a reducing agent in a solvent and inthe presence of a stabilizer, in order to form a suspension containingcolloidal particles of platinum or platinum/ad-metal;

b) mixing the resulting suspension with a co-solvent;

c) subjecting the resulting mixture to centrifugal treatment, therebygiving rise to platinum or platinum/ad-metal colloidal particle portionand a liquid portion;

d) repeating the co-solvent mixing and centrifugal treatment to theplatinum or platinum/ad-metal colloidal particle portion, until theresulting liquid portion substantially does not contain a product of thereduction reaction; and

e) drying the platinum or platinum/ad-metal colloidal particle portionto obtain a platinum based nano-size catalyst.

2. The method of Item 1, wherein the co-solvent used in step b) isacetone, C₃-C₁₂ aliphatic alkane or toluene.

3. The method of Item 2, wherein the C₃-C₁₂ aliphatic alkane is pentane.

4. The method of Item 1, wherein the solvent, reducing agent, andstabilizer used in step a) is ethylene glycol.

5. The method of Item 2, wherein the solvent, reducing agent, andstabilizer used in step a) is ethylene glycol.

6. The method of Item 3, wherein the solvent, reducing agent, andstabilizer used in step a) is ethylene glycol.

7. The methods of Item 1, wherein in step a), the solvent is a mixedsolution of H₂O and ethanol, the reducing agent is ethanol, and thestabilizer is polyvinyl pyrrolidone (PVP).

8. The methods of Item 2, wherein in step a), the solvent is a mixedsolution of H₂O and ethanol, the reducing agent is ethanol, and thestabilizer is polyvinyl pyrrolidone (PVP).

9. The methods of Item 3, wherein in step a), the solvent is a mixedsolvent of H₂O and ethanol, the reducing agent is ethanol, and thestabilizer is polyvinyl pyrrolidone (PVP).

10. The method of Item 4, wherein the platinum precursor used in step a)is chloroplatinic acid (H₂PtCl₆), and the product of the reductionreaction in step d) is chloride ions.

11. The method of Item 7, wherein the platinum precursor used in step a)is chloroplatinic acid (H₂PtCl₆), and the product of the reductionreaction in step d) is chloride ions.

12. The method of Item 10, in which the liquid portion in step d)contains less than 300 ppm of chloride ions.

13. The method of Item 11, in which the liquid portion in step d)contains less than 300 ppm of chloride ions.

14. The method of Item 1, wherein the platinum precursor used in step a)is chloroplatinic acid (H₂PtCl₆), bis(acetylacetonato) platinum (II),dinitrodiaminoplatinum (II) (Pt(NH₃)₂(NO₂)₂), or tetraaminoplatinum(II).

15. The method of Item 1, wherein the solvent used in step a) isethylene glycol or a mixed solvent of H₂O and alcohol.

16. The method of Item 1, wherein the reducing agent used in step a) isethanol, ethylene glycol, sodium borohydride, hydrazines, orformaldehyde.

17. The method of Item 1, wherein the stabilizer used in step a) ispolyvinyl pyrrolidone (PVP), polyol, polyvinyl alcohol, or sodiumbis(2-ethylhexyl) sulfosuccinate (AOT).

The present invention can be more fully comprehended by reading thefollowing examples, which only serve as examples and are not to be usedto limit the scope of the present invention.

EXAMPLE 1

0.125 g of chloroplatinic acid (H₂PtCl₆) was dissolved in 250 ml ofH₂O/ethanol solution, in which the volume ratio between H₂O and ethanolis 2:3. Then polyvinyl pyrrolidone (PVP) was added, in which the weightratio between platinum and PVP was 0.12:1. After sonicating theresulting mixture for 30 minutes, it was then heated to and maintainedat 90° C. for 1 hour, thereby giving rise to a suspension of colloidalparticles of platinum. After the heating was ceased, the suspension waskept at room temperature for 1 year, in which colloidal particles stillremained without precipitation. The suspension was mixed with 150 ml ofa mixed solvent of ethylene glycol/acetone, wherein the volume ratiobetween ethylene glycol and acetone is 1:6. The mixture was thensubjected to sonication for 30 minutes followed by centrifugaltreatment; and the liquid portion resulting from the centrifugaltreatment was analyzed by using an ion chromatography (IC) in order tomeasure the amount of chloride therein. The ethylene glycol/acetonesolvent mixing (the volume used for mixing was 300 ml) and centrifugaltreatment were repeated to the solid portion resulting from thecentrifugal treatment, until the amount of chloride contained in theliquid portion resulting from centrifugal treatment measured by the ICanalysis was lower than 300 ppm. The number of the repeating was sixtimes in total. The solid portion resulting from the last centrifugaltreatment was dried by baking (110° C., 12 hours), which gave rise toplatinum black catalyst. Its average particle diameter of the platinumblack catalyst is 2.3 nm, which was obtained by the use of atransmission electron microscope. The activity of the catalyst preparedin this Example was tested by electrocatalytic oxygen reduction reaction(ORR) with a rotating disc electrode (RDE), and the current density wasfound to be 6.3 mA/mg Pt (Table 1).

EXAMPLE 2

1.935 g of chloroplatinic acid (H₂PtCl₆) was dissolved in 150 ml ofethylene glycol solution, then its pH value was adjust to between12˜12.2 by using a concentrated NaOH/EG solution, and the pH-adjustedsolution was then subjected to sonication for 30 minutes followed byheating to 140° C. and maintained at this temperature for 1 hour,thereby resulting in a suspension of colloidal particles of platinum.After the heating was ceased, the suspension was kept at roomtemperature for 1 year, in which colloidal particles still remainedwithout precipitation. The suspension was mixed with 150 ml of a mixedsolvent of ethylene glycol/pentane, wherein the volume ratio betweenethylene glycol and pentane is 1:6. The mixture was then subjected tosonication for 30 minutes followed by centrifugal treatment. Repeat theethylene glycol/pentane solvent mixing (the volume used for mixing is300 ml) and centrifugal treatment to the solid portion resulting fromthe centrifugal treatment, until the amount of chloride contained in theliquid portion resulting from the centrifugal treatment, which wasmeasured by IC analysis, was lower than 300 ppm (repeated for a total of6 times). The solid portion resulting from the last centrifugaltreatment was dried by baking (110° C., 12 hours), which subsequentlygave rise to platinum black catalyst. By the analysis of a transmissionelectron microscope, the average particle diameter of the platinum blackcatalyst was found to be 2.2 nm, as shown in FIG. 1. The activity of thecatalyst prepared in this Example was tested by electrocatalytic ORRwith RDE, and the current density was found to be 10.6 mA/mg Pt.

EXAMPLE 3

1.935 g of chloroplatinic acid (H₂PtCl₆) was dissolved in 150 ml ofethylene glycol solution, then its pH value was adjust to between12˜12.2 by using a concentrated NaOH/EG solution, and the pH-adjustedsolution was then subjected to sonication for 30 minutes followed byheating to 140° C. and maintained at this temperature for 1 hour,thereby resulting in a suspension of colloidal particles of platinum.After the heating was ceased, the suspension was kept at roomtemperature for 1 year, in which colloidal particles still remainedwithout precipitation. The suspension was mixed with 150 ml of a mixedsolvent of ethylene glycol/toluene, wherein the volume ratio betweenethylene glycol and toluene is 1:6. The mixture was then subjected tosonication for 30 minutes followed by centrifugal treatment. Repeat theethylene glycol/toluene solvent mixing (the volume used for mixing is300 ml) and centrifugal treatment to the solid portion resulting fromthe centrifugal treatment, until the amount of chloride contained in theliquid portion resulting from the centrifugal treatment, which wasmeasured by IC analysis, was lower than 300 ppm (repeated for a total of6 times). The solid portion resulting from the last centrifugaltreatment was dried by baking (110° C., 12 hours), which subsequentlygave rise to platinum black catalyst. By the analysis of a transmissionelectron microscope, the average particle diameter of the platinum blackcatalyst was found to be 2.2 nm. The activity of the catalyst preparedin this Example was tested by electrocatalytic ORR with RDE, and thecurrent density was found to be 10.0 mA/mg Pt.

TABLE 1 The comparison of catalyst activity TEM Current particle DensityPt/EG PVP Temp. Time diameter (mA/mg Example (g/ml) (g) Co-solvent (°C.) (Hr) (nm) Pt@0.9 V) Ex. 1 0.125/0  1.07 EG/acetone  90 1 2.3 6.3 Ex.2 1.943/150 — EG/pentane 140 1 2.2 10.6 Ex. 3 1.943/150 — EG/toluene 1401 2.2 10.0 JMBlack¹ — — — — — ~5 5.4 ¹JMBlack: Johnson Matthey Company,number HiSPEC 1000 platinum catalyst.

1. A method for preparing a platinum based nano-size catalyst comprisingthe following steps: a) carrying out a reduction reaction of a platinumprecursor and an optional ad-metal precursor with a reducing agent in asolvent and in the presence of a stabilizer, in order to form asuspension containing colloidal particles of platinum orplatinum/ad-metal; b) mixing the resulting suspension with a co-solvent;c) subjecting the resulting mixture to centrifugal treatment, therebygiving rise to platinum or platinum/ad-metal colloidal particle portionand a liquid portion; d) repeating the co-solvent mixing and centrifugaltreatment to the platinum or platinum/ad-metal colloidal particleportion, until the resulting liquid portion substantially does notcontain a product of the reduction reaction; and e) drying the platinumor platinum/ad-metal colloidal particle portion to obtain a platinumbased nano-size catalyst.
 2. The method of claim 1, wherein theco-solvent used in step b) is acetone, C₃-C₁₂ aliphatic alkane ortoluene.
 3. The method of claim 2, wherein the C₃-C₁₂ aliphatic alkaneis pentane.
 4. The method of claim 1, wherein the solvent, reducingagent, and stabilizer used in step a) is ethylene glycol.
 5. The methodof claim 2, wherein the solvent, reducing agent, and stabilizer used instep a) is ethylene glycol.
 6. The method of claim 3, wherein thesolvent, reducing agent, and stabilizer used in step a) is ethyleneglycol.
 7. The methods of claim 1, wherein in step a), the solvent is amixed solution of H₂O and ethanol, the reducing agent is ethanol, andthe stabilizer is polyvinyl pyrrolidone (PVP).
 8. The methods of claim2, wherein in step a), the solvent is a mixed solution of H₂O andethanol, the reducing agent is ethanol, and the stabilizer is polyvinylpyrrolidone (PVP).
 9. The methods of claim 3, wherein in step a), thesolvent is a mixed solvent of H₂O and ethanol, the reducing agent isethanol, and the stabilizer is polyvinyl pyrrolidone (PVP).
 10. Themethod of claim 4, wherein the platinum precursor used in step a) ischloroplatinic acid (H₂PtCl₆), and the product of the reduction reactionin step d) is chloride ions.
 11. The method of claim 7, wherein theplatinum precursor used in step a) is chloroplatinic acid (H₂PtCl₆), andthe product of the reduction reaction in step d) is chloride ions. 12.The method of claim 10, in which the liquid portion in step d) containsless than 300 ppm of chloride ions.
 13. The method of claim 11, in whichthe liquid portion in step d) contains less than 300 ppm of chlorideions.
 14. The method of claim 1, wherein the platinum precursor used instep a) is chloroplatinic acid (H₂PtCl₆), bis(acetylacetonato) platinum(II), dinitrodiaminoplatinum (II) (Pt(NH₃)₂(NO₂)₂), ortetraaminoplatinum (II).
 15. The method of claim 1, wherein the solventused in step a) is ethylene glycol or a mixed solvent of H₂O andalcohol.
 16. The method of claim 1, wherein the reducing agent used instep a) is ethanol, ethylene glycol, sodium borohydride, hydrazines, orformaldehyde.
 17. The method of claim 1, wherein the stabilizer used instep a) is polyvinyl pyrrolidone (PVP), polyol, polyvinyl alcohol, orsodium bis(2-ethylhexyl) sulfosuccinate (AOT).